Schottky barrier detectors are used as optical detectors in night vision applications, for example, in which photons in the far infrared range, typically in the 10 micron wavelength region, are detected. Typically, a semiconductor such as silicon is employed with an overlying Schottky electrode of a metal material chosen to provide a very low (0.1–0.2 ev) Schottky barrier energy across the metal-semiconductor Schottky interface. In a Schottky detector, the incoming light is incident on the overlying metal Schottky electrode, in a direction generally normal to the electrode surface. Electrons in the metal electrode absorb incident photons and are thereby given sufficient energy to cross the Schottky energy barrier (between the metal and semiconductor layers) and flow into the semiconductor layer, resulting in a photo current across the device.
Schottky barrier detectors have not been generally employed in optical communication systems, which tend to operate at shorter wavelengths in the range of about 1 to 3 microns. This is because a Schottky detector is typically less efficient than other types of detectors in this wavelength range. And, detector efficiency in a long-range optical communication system is critical.
The efficiency of a Schottky barrier detector is determined by a number of factors, including the geometry of the electrode or metal layer. This is because the incident light, generally normal to the electrode surface, is absorbed by electrons during its passage through the thin metal layer. The probability of absorbing all incident photons is enhanced in a thicker electrode layer. But, as the metal electrode layer thickness is increased, the number of photo-excited electrons that actually reach the semiconductor-metal interface is reduced by collision losses within the metal electrode. Thus, optimizing photon absorption in the Schottky electrode is difficult and depends upon controlling the very small dimension of the Schottky electrode thickness.
In summary, it has not seemed practical to use a Schottky detector for optical communications because high detector efficiency is so important, and because controlling detector efficiency requires difficult or expensive process control over the Schottky electrode thickness.
The invention that will be described below in this specification arose during consideration by the inventors herein of the problem of integrating an optical waveguide and an optical detector in a planar integrated circuit as a locally distributed optical communication system operating at a short wavelength (e.g., 1.3 microns). A locally distributed optical communication system envisioned by the inventors herein could distribute a global clock signal in a VLSI integrated circuit (for example) via a network of optical waveguides, to reduce power consumption associated with conventional electrical distribution of the clock signal. One problem is that in such a system, the waveguide would be complex (e.g., it might need a number of terminations equal to the number of detectors to be fed by the waveguide). A solution needed to be found that would provide a very simple and easily fabricated waveguide/detector structure.
One problem with a conventional Schottky barrier detector integrated with a waveguide, such as that described in U.S. Pat. No. 4,857,973, is that its structure cannot be easily combined in a planar integrated circuit structure. This is because such a device is not planar, in that the waveguide is a rib above the plane of the integrated circuit. This feature is necessary in order to establish a difference in refractive index around the rib that confines light travelling through the rib. Such a structure cannot be combined in a planar VLSI integrated circuit because the fabrication and performance of such a circuit requires a planar structure. In a planar structure, structural features in the semiconductor substrate are below the top surface of the substrate, the surface of the substrate being reserved for overlying insulator layers and such features as conductor lines, and, in some cases, silicon-on-insulator transistor structures, for example.
The problem is how to integrate a Schottky barrier detector with a waveguide in a planar VLSI integrated circuit.